1. Field of the Invention
This invention relates to a slip assembly that can be used to press against the inside wall of a tubular to anchor a tool in the tubular without significantly deforming or damaging the wall, even at high anchoring force, and the use of the slip assembly in a bridge plug or other device to be anchored in a tubular.
2. Description of Related Art
Slips are any self-gripping device consisting of three or more wedges that are held together and form a near circle either (1) around an object to be supported by contact with surfaces of the slips or (2) within a tubular to anchor an object within the tubular. The first type of slips is normally used to grip a drill string, wire line or other cylindrical devices suspended in a well. The second type of slips is used to anchor bridge plugs, frac plugs, cement retainers and other devices temporarily or permanently placed at a selected location within tubulars. Normally, the slips are fitted with replaceable, hardened tool steel teeth that embed into the outside or inside surface of the tubular.
The embedment of the hardened steel teeth of slips causes permanent damage to the outside or inside surface of tubulars. Linear or non-linear notches may be formed that can cause stress concentration in the tubular wall. Under some conditions the damage is inconsequential, but under other conditions, such as when high-strength or corrosion-resistant pipe is used, the damage may lead to stress cracking or stress failure of the tubular.
A slip assembly consists of slips and a cone to displace the slips either radially inward (first type of slip assembly) or radially outward (second type of slip assembly). In the second type of slip assembly, a cone slides along the inside surface of the slips, pressing them radially outward, as the cone moves axially along a mandrel within the slips. The applications of slip assemblies disclosed herein use the second type of slip assembly.
One of the applications of the second type of slip assembly is a bridge plug or a special type of bridge plug called a “frac plug.” The bridge plug may be set in the casing of a well by wireline, coiled tubing or conventional pipe. The plug is often set by attaching it to a wireline setting tool. The setting tool may include a latch-down mechanism and a ram. The plug is lowered through the casing to a desired location, where the setting tool is activated. The setting tool pushes a cone on a mandrel axially, forcing a slip (or two slips if the plug is to hold in both directions) into contact with the inside wall of the casing. A sealing element, normally made from an elastomer, is then pushed radially outward to contact the inside wall of the casing. Increasing fluid pressure differential across the bridge plug normally increases the sealing force. There is a need for a slip assembly that does not damage the inside wall of casing when it is set.
Some bridge plugs are not retrievable because the slips are not designed to release and retract but to be removed by milling or drilling. The slips alone may be milled, releasing the plug to be pushed or pulled along the casing, or in some applications it is desirable to remove the entire plug by drilling or milling it to form cuttings of a size that can be removed from the casing by flow of fluid. The time required to mill or drill a bridge plug from a well is very important, particularly when the bridge plug is used in high-cost operations or when multiple bridge plugs are set in a casing for fracturing multiple intervals along a horizontal section of a well. Therefore, the plug should preferably be made of a material that drills easily. Also, it is often important to remove the plug without damaging the inside wall of the casing. A mill or drill bit may be used to reduce the components of the bridge plug to a size such that they can be circulated from the wellbore by drilling fluid. Since a conventional junk mill will normally damage the inside surface of casing, it is preferable to use a bit, such as a PDC bit, that has a smooth gage surface, to avoid casing damage. In prior art bridge plugs, it has been found that lower components of the bridge plug may no longer engage the mandrel during drilling or milling of the plug, allowing them to spin or rotate within the casing and greatly increase the time required for drilling. Interlocking surfaces at either end of a bridge plug are needed to allow drilling of multiple bridge plugs without rotation. Accordingly, for maximum value, a bridge plug is needed that can be drilled quickly, with a bit that does not damage the surface of casing and that can be stacked for drilling of multiple plugs without rotating.